<p>We studied the east intersection between the Romanche transform fault (TF) and the Mid-Atlantic ridge using bathymetry and gravity anomalies, to investigate the temporal evolution of the ultra-cold ridge-transform intersection. Our results reveal a complex ridge axis, with evidence of a significant decrease in the along-axis melt supply towards the RTI but also since ~10 Ma.</p> <p>Over a 100 km distance south of the RTI, the ridge axis is formed by three spreading segments offset by large non-transform discontinuities. Large detachment faults mark the present-day spreading style at the RTI, while magma supply increases away from the Romanche intersection. Axial and near-axis fault patterns reveal a marked obliquity, especially in the north and center of the study area.In lithosphere older than 10 Ma, the ridge axis appears to form a single spreading segment between the Romanche and Chain TFs, perpendicular to the spreading direction, with relatively regular abyssal hills. From around 10 to 3 Ma, oceanic core complexes (OCCs) developed in the northern part of the ridge axis south of the Romanche TF.  The complexity of the ridge axis appears to have increased in the last 3 Ma, with ridge obliquity accompanying axial instabilities and ridge jumps.  At least three eastward ridge relocations were identified immediately south of the Romanche TF, rupturing a series of OCCs located in the African plate, east of the ridge axis. This pattern could reflect a progressive decrease in the melt supply, in particular since 3-5 Ma. This may be related to a significant reduction of the ridge spreading rate as seen from kinematic models which allowed the cooling effect of the large offset Romanche TF to dominate the spreading processes in the area.</p>

<p>At mid-oceanic ridges, mantle temperature and magma supply influence the structure of the neo-volcanic zone. Due to the large Romanche offset, a strong “cold edge” effect is present at its eastern intersection with the Mid-Atlantic Ridge. This effect decreases with the distance from the transform fault, making this region an ideal area to study the impact of the thermal gradient on the architecture of the neo-volcanic zone. We analyzed seafloor videos and photos from submersible dives, as well as bathymetry and backscatter data collected during the SMARTIES cruise (2019), from the Ridge-Transform Intersection (RTI) to approximately 80 km to the south of it. We produced maps at local and regional scales and quantified the morphology of volcanoes (height, diameter, height/diameter ratio, volume and surface). Visual observations have showed that the seafloor is mainly made up of pillows or elongated pillows and rare massive lava flows. Within 30 km of the RTI, the neo-volcanic area is characterized by clusters of volcanoes, affected by faults trending N120-130° E, oblique to the extension and to the transform fault orientation, and by faults trending E-W. At 30 km to 50 km from the RTI, the Central segment displays a robust volcanic ridge oriented N150°E built by a pilling of volcanoes and narrow ridges. Its eastern and southern parts are old and characterized by oblique faults and narrow ridges (N130-140°E), while the northwest portion is more recent, faults and ridges are almost normal to the extension. The southernmost segment, located at 80 km from the RTI, is orthogonal to the spreading direction and asymmetric, bounded at the west by a detachment fault. Recent volcanic edifices were observed from the center of the segment to the base of the detachment. Our observations suggest that the neo-volcanic area is fed by more and more magma from north to south. This increase in magma supply is marked by a more structured volcanic axis, volcanoes that are more voluminous and a change in the orientation of the segments and faults. Changes in the orientation of faults and off-axis abyssal hills also reveal variations in the magmatic supply over time.</p>

Oceanic crust is formed by melt derived from the mantle at oceanic spreading centers. A small amount of melting initiates at about 150-300 km depths in the presence of volatiles (CO2, H2O)1–3, but the extensive dry melting commences at 60-70 km depths due to the upwelling of the mantle as two diverging plates move apart4,5. However, how these melts migrate to the surface and what happens to these melts in the upper part of the mantle are still not understood. Using seismological data recorded by ocean-bottom seismometers, here we report the presence of deep earthquakes at 10-20 km depth in the hot mantle along the Mid-Atlantic Ridge axis, much below the brittle-ductile boundary, suggesting that these earthquakes are caused by a volume change associated with the CO2 degassing from the ascending melt. The geochemical analyses of basalts from the ridge axis show an abnormally high quantity of CO2 (>1.9 wt%) in the primitive melt, confirming the CO2 degassing hypothesis. The large concentration of CO2 in the primitive melt will influence the presence of melt beneath the lithosphere-asthenosphere boundary at sub-solidus temperatures.